In many receivers, and especially in a high performance multi carrier receiver, the linearity and dynamic range requirements are very challenging. To meet the demanding requirements, automatic gain control (AGC) can be used to adjust the dynamic range of the receiver according to the received signal level and thus relax the dynamic range of the receiver circuitry. In such a receiver an AGC control function is needed.
FIG. 1 schematically shows the principles of a prior art AGC receiver 100 with an input terminal 101 and an output terminal 102. The components coupled in series between the input and output terminals are part of an Rx-chain. An input signal at input terminal 101 passes an attenuator 103, an amplifier 104, a band pass filter 105 and an Analogue Digital Converter (ADC), 106. After the ADC the signal is divided in a first branch 109 entering an AGC unit 107 and a second branch 110 entering an AGC compensation unit 108. The AGC unit 107 includes one high and one low threshold detector for detecting if the signal in the first branch entering the AGC unit 107 exceeds one predetermined high threshold level or is below one predetermined low threshold level. The AGC unit 107 initiates an increase or decrease of the attenuation in the attenuator 103 through a control signal via a connection 111. When a low threshold level is detected the attenuation will be decreased and when a high threshold level is detected the attenuation will be increased. In order to have the same signal level at the input terminal 101 and the output terminal 102, a further control signal from the AGC unit via a connection 112 will initiate a compensating gain increase or decrease at an AGC compensation unit 108. When the attenuator decreases attenuation, the AGC compensation unit will decrease the gain and when the attenuator increases the attenuation the AGC compensation unit will increase the gain.
The AGC unit 107 according to prior art is shown in FIG. 2. The AGC unit 107 consists of a high, 201, and a low, 202, threshold detector which controls the AGC level up or down. The AGC control unit, 203, controls the analogue attenuation and the AGC compensation. The AGC input signal 204 is divided in two paths, one fed to the high threshold detector for detection of a high threshold level and one to the low threshold detector for detection of a low threshold level. When the high threshold level is detected the high threshold detector sends a detection signal to the AGC control unit 203 and when a low threshold level is detected the low threshold detector sends a further detection signal to the AGC control unit. The AGC control unit produces the two control signals mentioned above in association with FIG. 1. When the high threshold level is detected the control signal informs the attenuator 103 to increase attenuation and the further control signal informs the AGC compensation unit 108 to increase gain to compensate for the attenuation. This means that the attenuation “AGC attenuation” is compensated with the same amount of gain increase “AGC compensation” and the level of the input signal at the input terminal 101 will be equal to the level of the AGC compensation unit output signal at the output terminal 102. When the low threshold level is detected the control signal informs the attenuator 103 to decrease attenuation and the further control signal informs the AGC compensation unit 108 to decrease gain to compensate for the decreased attenuation.
In an AGC receiver, it is important that the gain always is as high as possible without limiting the received signal in order to get the highest possible Signal to Noise Ratio (SNR) and thus the best performance. All AGC transitions, i.e. changes in the receiver gain controlled by the AGC unit, will however generate distortion due to imperfections between the gain change and the gain compensation circuitries.
For signals with Gaussian signal distribution, and thus a high Peak to Average Ratio (PAR), it is difficult to minimize the number of transitions and it has been necessary either to increase the hysteresis and/or to have a long integration time to estimate the signal level accurately for the lower AGC threshold, as will be explained in FIG. 3. The PAR value is defined as the envelope peak power value in relation to the average power value of the signal. A large hysteresis and/or a long time interval without peaks above the threshold level will degrade the performance in the receiver since the gain will not be maximized all the time. Today signals with close to Gaussian distribution are very common for new communication systems as for example in mobile telephone systems as GSM, CDMA, WCDMA or LTE. (GSM=Global System for Mobile communication, CDMA=Code Division Multiple Access, WCDMA=Wideband Code Division Multiple Access, LTE=Long Term Evolution)
FIG. 3 shows a high PAR signal 303 and a low PAR signal 304 as a function of time t on the horizontal axis 301 and signal power P on the vertical axis 302. The low threshold level 305 is marked on the power axis. To minimize the number of AGC transitions, i.e. the number of instances when the attenuation level is changed, a relatively long detection interval 306 has to be chosen. With the prior art solution of today the signal has to be below the threshold level 305 during the detection interval 306 before the attenuation is released and thus the gain of the analogue part of the AGC receiver is increased. This means that the analogue part of the receiver is working at a reduced gain during the relatively long detection interval 306. This will as mentioned above degrade the performance of the receiver since the gain will not be maximized at all times.
There is thus a need for an improved utilization of the dynamic range of the receiver by an improved solution for low threshold level detection that will allow the gain to be maximized during a longer part of the total operating time without increasing the number of AGC transitions.